A two piece can which consists of the can body combined with the bottom lid is used for the manufacture of a food can or beverage can. Those cans are formed from a metal substrate such as a cold rolled steel sheet, an aluminum sheet or tinplate. Those cans are usually coated with various resin coatings in order to prevent the metal from being dissolved into the contents of the can and to improve its corrosion resistance and durability.
The above-mentioned two-piece can is occasionally formed by means of drawing the laminated sheet which is produced by laminating the resin film to the metal substrate. Such laminated sheet should endure the drawing, the pressing or the stretching processing.
Therefore, the laminated sheet is conventionally produced by a method of laminating the resin film without using adhesive (Japanese patent Application No. TOKU-KOU-SHO 60-47103) or by a method of laminating a polyester film which is coated with epoxy resin together with curing agent, etc. (e.g. Japanese Application NO. TOKU-KOU-SHO 63-13829 or No. TOKU-KAI-HEI 1-249331).
One of the requirements is denting resistance in the drawn and formed can. There must be impact resistance of the film and also resistance to cracking of the film when the can is dented.
We have previously proposed to use the steel sheet covered with bi-axially oriented or un-oriented polyester resin film which has a specified intrinsic viscosity as a material of the laminated can (Japanese Patent Application No. TOKU-KAI-HEI 4-224936).
Above-mentioned bi-axially oriented polyester resin film improves the strength and the denting resistance because the crystalline structure is highly oriented (the molecule is highly oriented). However, such a polyester film does not have enough adhesion to the metal substrate. Therefore it peels off easily during drawing process or ironing (e.g. pressing) process.
In order to solve such problems, the resin film is laminated to the metal substrate which is heated in advance. As a result, the crystal orientation of the film on the side of the oriented film in contact with the metal substrate decreases to some degree. And, adhesion and bonding improve.
So this is a method of giving the inclination in the direction of thickness of the film concerning crystal orientation. However, the denting resistance of the can decreases when crystal orientation of the film decreases too much and it causes a problem concerning the durability of the can.
In order to improve adhesion, it has been proposed that the film of laminated sheet be partially heated, especially for the high adhesion required part, just before the drawing process (Japanese patent application No.TOKU-KAI-HEI 4-118121). However, it is difficult to control and maintain the film temperature within the range from the glass transition temperature to the temperature of re-crystallization.